One of the conventional configurations of fuel cell systems capable of detecting cross-leakage of electrolyte membrane is described in Patent Literature 1. In the fuel cell system described in Patent Literature 1, the pressure of anode gas during operation is set higher than the pressure of cathode gas to detect cross-leakage based on a drop in voltage in an activated overvoltage region. Such detection of cross-leakage is excellent to diagnose a fuel cell which is operating.